Field of the Invention
The present invention relates to anti-skid or anti-slip brake control systems.
In vehicles that are equipped with an anti-skid or anti-slip brake control system, typically four and in individual cases only three rpm sensors are used, which are assigned to the individual wheels. In general, the output signals of these rpm sensors are pulselike. From the time interval between individual pulses or individual pulse edges, the rpm of the applicable wheel is ascertained.
The vehicle reference speed, which represents an essential variable for the anti-skid or anti-slip brake control is ascertained from the wheel rpm values. The vehicle reference speed is ascertained for instance from the four wheel speeds or from the wheel speed of one wheel, the so-called reference i wheel. From the wheel rpm values, a reference speed can also be formed, which is usable in the ABS control as a further controlled variable.
Since essential variables of the anti-skid or anti-slip brake control system are formed on the basis of output signals of the wheel rpm sensors, reliable monitoring of the operability of the sensors must necessarily be performed. German Patent Application P 44 05 801.2, for instance, has therefore proposed a monitoring system that is used in ABS control systems and makes it possible to detect various malfunctions of the wheel rpm sensors.
Interference signals that correspond approximately to the mains frequency of 50 Hz (or 60 Hz) have proved to be especially problematic in control systems. Such interference signals can be fed to the rpm sensors of the ABS system for instance by magnetic fields generated by vehicle heaters that are operated at the usual mains voltage. In that event, they reflect incorrect wheel speeds to the control unit, and as a result an unintended ABS control can be tripped and maintained.